Curable aqueous polyurethane dispersions

ABSTRACT

A curable aqueous polyurethane dispersion substantially comprises
     a) at least one compound having two free isocyanate groups, at least one allophanate group and at least one C═C double bond capable of free radical polymerization, a carbonyl group or an oxygen atom in the form of an ether function being bonded directly to the double bond,   b) at least one compound having at least one group reactive toward isocyanate groups and having a C═C double bond capable of free-radical polymerization,   c) if required, at least one compound having at least two groups reactive towards isocyanate groups and selected from hydroxyl, mercapto, primary and/or secondary amino groups,   d) at least one compound having at least one group reactive towards isocyanate groups and having at least one acid group,   e) at least one basic compound for neutralizing or partially neutralizing the acid groups of the compounds d),
       and a thermal initiator and is used for coating substrates.

The present invention relates to UV-curable and heat-curablepolyurethane dispersions, a process for their preparation and their use.

Radiation-curable polyurethane dispersions are disclosed, for example,in DE-A-44 34 554 and are prepared from polyisocyanates,hydroxyl-containing polyesters, compounds having a group reactive towardisocyanates and having an acid group and compounds having a groupreactive toward isocyanates and having C═C double bonds. However, theproducts are unsatisfactory with regard to their processability.

U.S. Pat. No. 5,859,135 describes aqueous coating mixtures comprising alipophilic polymer having at least one hydroxyl group and a molecularweight of up to 100 000, which is bonded on the one hand to acrosslinkable functional group and on the other hand, via apolyisocyanate, to a group having carboxyl groups and to a hydrophilicpolyalkylene oxide monoether radical. The disadvantage is that coatingsproduced therewith have relatively pronounced intrinsic hydrophilicproperties even after processing.

U.S. Pat. No. 5,296,529 describes a self-crosslinking resin havingcarboxyl groups, hydroxyl groups and blocked isocyanate groups, which isprepared from a) a copolymer of a vinyl monomer having free and blockedisocyanate groups with a styrene and/or (meth)acrylate comonomer and b)a hydroxyl- and carboxyl-containing polyester resin, some hydroxylgroups of the polyester resin being reacted with some free isocyanategroups of the vinyl copolymer and the remaining isocyanate groups thenbeing blocked. It is clear that the risk of premature undesiredcrosslinking of the two polymers may prevent readily reproducibleproduction of the system. Furthermore, the system described contains noUV-curable double bonds, and no radiation curing thereof is described.

German patent application P 199 47 054.5 describes UV-curable andheat-curable polyurethane dispersions based on aliphaticpolyisocyanates, it also being possible for said polyisocyanates to bepolyisocyanates containing allophanate groups. These dispersionsinevitably contain isocyanate groups blocked with an isocyanate blockingagent.

DE-A-198 60 041 describes reaction products of a) polyisocyanates and b)low molecular weight hydroxyl compounds having C═C double bonds, such ashydroxyalkyl (meth)acrylates or hydroxyalkyl vinyl ethers, which for themost part are allophanates of the polyisocyanates with the unsaturatedalcohols. The low molecular weight and low-viscosity reaction productshave a high content of C═C double bonds in the molecule and can be curedboth with UV radiation and with participation of the isocyanate groups,for example by the action of steam, ammonia or amines. A use in the formof aqueous dispersions is not described.

It is an object of the present invention to provide heat-curable andalso UV-curable aqueous polyurethane dispersions. These should givecoatings having good performance characteristics, such as goodresistance to chemicals and good mechanical properties, in particularthey should have high scratch resistance and, after treatment, shoulddry physically also in unexposed parts and should be suitable forexterior applications, such as automotive coatings.

We have found, surprisingly, that this object is achieved by curableaqueous polyurethane dispersions comprising

-   a) at least one compound having at least two free isocyanate groups,    at least one allophanate group and at least one C═C double bond    capable of free radical polymerization, a carbonyl group or an    oxygen atom in the form of an ether function being bonded directly    to the double bond,-   b) at least one compound having at least one group reactive toward    isocyanate groups and having a C═C double bond capable of    free-radical polymerization,-   c) if required, at least one compound having at least two groups    reactive toward isocyanate groups and selected from hydroxyl,    mercapto, primary and/or secondary amino groups,-   d) at least one compound having at least one group reactive toward    isocyanate groups and having at least one acid group,-   e) at least one basic compound for neutralizing or partially    neutralizing the acid groups of the compounds d),-   f) if required, at least one compound differing from b), d) and e)    and having only one group reactive toward isocyanate groups,-   g) if required, at least one polyisocyanate differing from a),-   h) at least one thermal initiator,-   i) if required, further additives which are selected from reactive    diluents, photoinitiators and conventional coating additives, and-   k) water.

No compounds which contain isocyanate groups and in which some or all ofthe isocyanate groups have been reacted with blocking agents are used inthe novel dispersions. Blocking agents are understood as meaningcompounds which convert the isocyanate groups into blocked (capped orprotected) isocyanate groups which then do not exhibit the usualreactions of a free isocyanate group below the deblocking temperature.Such compounds not used according to the invention and having blockedisocyanate groups are usually used in dual-cure coating materials whichare subjected to final curing by curing involving the isocyanate groups.

After their preparation, the novel polyurethane dispersions preferablyhave substantially no free isocyanate groups.

Component a)

The novel compounds of component a) are preferably substantially free ofuretdione, biuret or isocyanurate groups.

The component a) is preferably selected from compounds of the formula IOCN—R¹

R²—C(O)—R²—R¹

_(n)NCO   (I)where

-   n is an integer from 1 to 10,-   R¹ is a divalent aliphatic or alicyclic C₂- to C₂₀-hydrocarbon unit    or an aromatic C₆- to C₂₀-hydrocarbon unit,-   R² in each repeating unit is —NH— on the one hand and, on the other    hand,

-   -   R³ being a radical derived from an alcohol A by abstraction of        the H atom from the alcoholic hydroxyl group, the alcohol A        additionally having at least one C═C double bond capable of free        radical polymerization, and a carbonyl group or an oxygen atom        being bonded by an ether bond directly to the double bond.

The radicals R¹ are preferably those which are derived by abstraction ofthe isocyanate group from conventional aliphatic or aromaticpolyisocyanates. The diisocyanates are preferably aliphatic isocyanatesof 4 to 20 carbon atoms. Examples of these conventional diisocyanatesare aliphatic diisocyanates, such as tetramethylene diisocyanate,hexylmethylene diisocyanate, octamethylene diisocyanate, decamethylenediisocyanate, dodecamethylene diisocyanate, tetradecamethylenediisocyanate, derivatives of lysine diisocyanate, tetramethylxylylenediisocyanate, cycloaliphatic diisocyanates, such as 1,4-, 1,3- or1,2-diisocyanatocyclohexane, 4,41- or2,4′-di(isocyanatocyclohexyl)methane, isophorone diisocyanate, 1,3- or1,4-bis(isocyanatomethyl)cyclohexane, 2,4- and2,6-diisocyanato-1-methylcyclohexane, and aromatic diisocyanates, suchas tolylene 2,4- or 2,6-diisocyanate, m- or p-xylylene diisocyanate,2,4′- or 4,4′-diisocyanatodiphenylmethane, phenylene 1,3- or1,4-diisocyanate, 1-chlorophenylene 2,4-diisocyanate, naphthylene1,5-diisocyanate, diphenylene 4,4′-diisocyanate,4,4′-diisocyanato-3,3′-dimethyldiphenyl diisocyanate,3-methyldiphenylmethane 4,4′-diisocyanate and diphenylether4,4′-diisocyanate. Mixtures of said diisocyanates may be present.Hexamethylene diisocyanate, 1,3-bis(isocyanatomethyl)cyclohexane,isophorone diisocyanate, tetramethylxylylene diisocyanate anddi(isocyanatocyclohexyl)methane are preferred.

The alcohols A from which R³ is derived are, for example, esters ofα,β-unsaturated carboxylic acids, such as acrylic acid, methacrylic acid(abbreviated to (meth)acrylic acid below), crotonic acid,acrylamidoglycolic acid, methacrylamido glycolic acid or vinyl aceticacid, and polyols having preferably 2 to 20 carbon atoms and at least 2hydroxyl groups, such as ethylene glycol diethylene glycol, triethyleneglycol, 1,2-propylene glycol, 1,3-propylene glycol, dipropylene glycol,tripropylene glycol, 1,4-butanediol, 1,5-pentanediol, neopentyl glycol,1,6-hexanediol, 2-methyl-1,5-pentanediol, 2-ethyl-1,4-butanediol,1,4-dimethylolcyclohexane, glycerol, trimethylolethane,trimethylolpropane, trimethylolbutane, pentaerythritol,ditrimethylolpropane, erythritol and sorbitol, provided that the esterhas at least one OH group reactive toward isocyanate. Furthermore, theradicals R³ may also be derived from the amides of (meth)acrylic acidwith amino alcohols, e.g. 2-aminoethanol, 3-amino-1-propanol,1-amino-2-propanol or 2-(2-aminoethoxy)ethanol, and the vinyl ethers ofthe abovementioned polyols, provided that they still have a free OHgroup.

Furthermore, unsaturated polyetherols or polyesterols or polyacrylatepolyols having a mean OH functionality of from 2 to 10 are also suitableas reactive components.

Preferably, the radicals R³ are derived from alcohols, such as2-hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate,1,4-butanediol mono(meth)acrylate, neopentylglycol mono(meth)acrylate,glyceryl mono- and di(meth)acrylate, trimethylolpropane mono- anddi(meth)acrylate, pentaerythrityl di- and tri(meth)acrylate. The alcoholA is particularly preferably selected from 2-hydroxyethyl acrylate,2-hydroxyethyl methacrylate, and hydroxypropyl (meth)acrylate. Examplesof amides of ethylenically unsaturated carboxylic acids with aminoalcohols are hydroxyalkyl (meth)acrylamides, such asN-hydroxymethyl(meth)acrylamide, N-hydroxyethyl(meth)acrylamide or5-hydroxy-3-oxopentyl(meth)acrylamide, N-hydroxyalkylcrotonamides suchas N-hydroxymethylcrotonamide, or N-hydroxyalkylmaleimides such asN-hydroxyethylmaleimide.

Component b)

The compounds of component b) have a C═C double bond capable of freeradical polymerization and at least one further group reactive towardisocyanate groups. Preferred compounds of component b) are, for example,the monoesters of dihydric or polyhydric alcohols with α,β-ethylenicallyunsaturated mono- and/or dicarboxylic acids and their anhydrides. Forexample, acrylic acid, methacrylic acid, fumaric acid, maleic acid,maleic anhydride, crotonic acid, itaconic acid, etc. may be used asα,β-ethylenically unsaturated mono- and/or dicarboxylic acids and theiranhydrides. Acrylic acid and methacrylic acid are preferably used.Suitable alcohols are, for example, diols, such as glycols, preferablyglycols of 2 to 25 carbon atoms, e.g. 1,2-ethanediol, 1,3-propanediol,1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,4-pentanediol,1,6-hexanediol, 1,10-decanediol, diethylene glycol etc. Suitable triolsand polyols have, for example 3 to 25, preferably 3 to 18, carbon atoms.These include, for example, glycerol, trimethylolpropane, erythritol,pentaerythritol, sorbitol, etc. The compounds of component b) arepreferably selected from 2-hydroxyethyl acrylate, 2-hydroxyethylmethacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate,3-hydroxybutyl acrylate, 3-hydroxybutyl methacrylate, 4-hydroxybutylacrylate, 4-hydroxybutyl methacrylate, 6-hydroxyhexyl acrylate,6-hydroxyhexyl methacrylate, 3-hydroxy-2-ethylhexyl acrylate,3-hydroxy-2-ethylhexyl methacrylate, trimethylolpropane monoacrylate andmixtures thereof. If desired, these compounds can also be subjected tochain extension by reaction with a suitable chain extender, for examplea polyfunctional isocyanate or a polyfunctional carboxylic acid.

Suitable compounds b) are furthermore the esters and amides of aminoalcohols with the abovementioned α,β-ethylenically unsaturated mono-and/or dicarboxylic acids, hydroxyalkyl vinyl ethers, such ashydroxybutyl vinyl ethers, etc.

Component c)

Suitable compounds c) are both low molecular weight alcohols andpolymeric polyols. Low molecular weight alcohols having a molecularweight of not more than 500 g/mol are preferred. Alcohols having from 2to 20 carbon atoms and 2 to 6 hydroxyl groups, for example theabovementioned glycols, are particularly preferred. Hydrolysis-stableshort-chain diols of 4 to 20, preferably 6 to 12, carbon atoms, areparticularly preferred. These preferably includedihydroxymethylcyclohexane, bis(hydroxycyclohexyl)propane,tetramethylcyclobutanediol, cyclooctanediol or norbornanediol. Aliphatichydrocarbon-diols such as the isomeric butanediols, pentanediols,hexanediols, heptanediols, octanediols, nonanediols, decanediols,undecanediols, and dodecanediols, are particularly preferred.1,4-butanediol, 1,4-pentanediol, 1,5-pentanediol, 1,6-hexanediol,2,5-hexanediol, dihydroxymethylcyclohexane,bis-hydroxycyclohexylpropane, etc. are particularly preferred.

Suitable compounds c) are furthermore polymeric polyols. The numberaverage molecular weight Mn of these polymers is preferably from about1000 to 100 000, particularly preferably from 2000 to 10 000. The OHnumbers are preferably from about 40 to 200 mg of KOH/g of polymer.Preferred polymers c) are, for example, copolymers which contain atleast one of the abovementioned monoesters of dihydric or polyhydricalcohols with at least one α,β-ethylenically unsaturated mono- and/ordicarboxylic acid and at least one further comonomer, preferablyselected from vinyl aromatics, e.g. styrene, esters of theabovementioned α,β-unsaturated mono- and/or dicarboxylic acids withmonoalcohols, vinyl esters of carboxylic acids of up to 20 carbon atoms,vinyl halides, nonaromatic hydrocarbons having 4 to 8 carbon atoms and 1or 2 double bonds, unsaturated nitriles, etc. and mixtures thereof aspolymerized units. These furthermore include (partially) hydrolyzedvinyl ester polymers, preferably polyvinyl acetates. These furthermoreinclude polyesterols based on aliphatic, cycloaliphatic and/or aromaticdi-, tri- and/or polycarboxylic acids with di-, tri- and/or polyols andlactone-based polyesterols. These furthermore include polyetherols,which are obtainable by polymerization of cyclic ethers or by reactionof alkylene oxides with an initiator molecule, and α,ω-diaminopolyethersobtainable by reaction of polyetherols with ammonia. These furthermoreinclude conventional polycarbonates known to a person skilled in the artand having terminal hydroxyl groups, which are obtainable, for example,by reaction of the abovementioned diols with phosgene or diesters ofcarboxylic acids.

The abovementioned components c) may be used individually or asmixtures.

Component d)

Preferably, the acid groups of the compounds of component d) areselected from carboxyl groups, sulfone groups, phosphonic acid groups,and phosphoric acid groups. Carboxyl and sulfo groups are preferred.

Particularly suitable compounds d) having at least one group reactivetoward isocyanates and having at least one carboxyl or sulfo group arealiphatic monomercapto-, monohydroxy- and monoamino- and iminocarboxylicacids and corresponding sulfonic acids, such as mercapto acetic acid(thioglycolic acid), mercaptopropionic acid, mercaptosuccinic acid,hydroxyacetic acid, hydroxypropionic acid (lactic acid), hydroxysuccinicacid, hydroxypivalic acid, dimethylolpropionic acid, hydroxydecanoicacid, hydroxydodecanoic acid, 12-hydroxystearic acid,hydroxyethanesulfonic acid, hydroxypropanesulfonic acid,mercaptoethanesulfonic acid, mercaptopropanesulfonic acid,aminopropanesulfonic acid, glycine (aminoacetic acid) or iminodiaceticacids.

Component e)

Suitable basic compounds e) for neutralizing or partially neutralizingthe acid groups of the compounds d) are inorganic and organic bases,such as alkali metal and alkaline earth metal hydroxides, oxides,carbonates, and bicarbonates and ammonia or primary, secondary ortertiary amines. Neutralization or partial neutralization with amines,such as ethanolamine or diethanolamine, and particularly with tertiaryamines, such as triethylamine, triethanolamine, dimethylethanolamine ordiethylethanolamine, is preferred. The amounts of chemically bonded acidgroups introduced and the degree of the neutralization of the acidgroups (which is generally from 40 to 100%, based on equivalents) shouldpreferably be sufficient to ensure dispersing of the polyurethanes in anaqueous medium, which is familiar to a person skilled in the art.

Component f)

In the novel dispersions, at least one further compound having a groupreactive toward isocyanate groups can be used as component f). Thisgroup may be a hydroxyl, mercapto or primary or secondary amino group.Suitable compounds f) are the conventional compounds which are known toa person skilled in the art and are usually used in the polyurethanepreparation as stoppers for reducing the number of reactive freeisocyanate groups or for modifying the polyurethane properties. Theyinclude, for example, monofunctional alcohols, such as methanol,ethanol, n-propanol, isopropanol, etc. Suitable components f) are alsoamines having a primary or secondary amino group, e.g. methylamine,ethylamine, n-propylamine, diisopropylamine, dimethylamine,diethylamine, di-n-propylamine, diisopropylamine, etc.

Component g)

In the novel dispersions at least one polyisocyanate differing from thecompounds of component a) may be used as components g). According to theinvention, no polyisocyanates in which the isocyanate groups have beenreacted with a blocking agent are used as component g).

Preferred compounds g) are polyisocyanates having an NCO functionalityfrom 2 to 4.5, particularly preferably from 2 to 3.5. Aliphatic,cycloaliphatic and araliphatic diisocyanates are preferably used ascomponent g). These include, for example, tetramethylene diisocyanate,hexamethylene diisocyanate, 2,3,3-trimethylhexamethylene diisocyanate,cyclohexylene 1,4-diisocyanate, isophorone diisocyanate, 1,3- and1,4-bis(isocyanatomethyl)cyclohexane, di-(isocyanatocyclohexyl)methane,tetramethylxylylene diisocyanate and mixtures thereof. Compounds g)which also have a group selected from the group consisting of theurethane, urea, biuret, allophanate, carbodiimide, uretonimine,uretdione and isocyanurate groups in addition to 2 or more isocyanategroups, are preferred.

Isophorone diisocyanate, 1,3- and 1,4-bis(isocyanatomethyl) cyclohexane,their isocyanurates, biurets and allophanates and mixtures thereof arepreferably used as component g).

If the novel dispersions also contain a component g) in addition to thecomponent a) the amount of the compound of component g) is preferablyfrom 0.1 to 90, particularly preferably from 1 to 50, in particular from5 to 30, % by weight, based on the total amount of the compounds ofcomponents a) and g).

Component h)

Preferred thermal initiators h) are those which have a half-life at 60°C. of at least one hour, preferably at least 2 hours. The half-life of athermal initiator is the time after which half the initial amount of theinitiator has decomposed into free radicals. These initiators generallypermit film formation on a substrate coated with a novel dispersion, byconventional methods, for example drying in the air with heating,substantially no thermal initiation and curing taking place as yet.

The component h) is preferably used in an amount of from 0.1 to 10,preferably from 0.5 to 5, % by weight, based on the total amount of thecomponents a) to i).

Suitable compounds h) are in general all compounds which decompose intofree radicals under the curing conditions, e.g. peroxides,hydroperoxides, hydrogen peroxide, persulfates, azo compounds, highlysubstituted, e.g. hexasubstituted, ethanes, amine-N-oxides, redoxcatalysts, etc. Water-soluble initiators are preferably used. Suitablethermal initiators are, for example, triphenylmethylazobenzene, benzoylperoxide, di-tert-butyl peroxide, cumyl hydroperoxide, dicumyl peroxide,tert-butyl perbenzoate, 2,2,6,6-tetramethylpiperidin-1-oxyl,benzopinacol and derivatives thereof, as well as (arylsulfonyl)aceticacid alkyl ester, preferably (phenylsulfonyl)acetic acid methyl ester.

Furthermore, the component h) preferably comprises at least one thermalinitiator having at least one group reactive toward isocyanate groups,the initiator, after reaction with a compound containing isocyanategroups, also being capable of liberating the radicals thermally. Theseinclude, for example, initiators which have at least one hydroxyl group,via which they can be incorporated into the polymer.

Hexasubstituted ethanes, in particular benzopinacol and the derivativesthereof, silylated pinacols, which are commercially available, forexample, under the tradename ADDID 600 from Wacker, orhydroxyl-containing amine N-oxides, such as4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL), are preferred.

Component i)

The novel dispersion may contain at least one further compound asusually used as a reactive diluent. Such compounds include, for example,reactive diluents as described in P. K. T. Oldring (Editor), Chemistry &Technology of UV & EB Formulations for Coatings, Inks & Paints, Vol. II,Chapter III: Reactive diluents for UV & EB Curable Formulations, Wileyand SITA Technology, London 1997.

Preferred reactive diluents are compounds which differ from thecomponent b) and have at least two functional groups which are selectedfrom C═C double bonds capable of free radical polymerization and groupsreactive toward isocyanate groups. These include, in particular, thediesters and polyesters of the abovementioned α,β-ethylenicallyunsaturated mono- and/or dicarboxylic acids with diols or polyols.Hexanediol diacrylate, hexanediol dimethacrylate, octanediol diacrylate,octanediol dimethacrylate, nonanediol diacrylate, nonanedioldimethacrylate, decanediol diacrylate, decanediol dimethacrylate,pentaerythrityl diacrylate, dipentaerythrityl tetraacrylate,dipentaerythrityl triacrylate, pentaerythrityl tetraacrylate, etc. areparticularly preferred. The esters of alkoxylated polyols withα,β-ethylenically unsaturated mono- and/or dicarboxylic acids, forexample the polyacrylates or polymethacrylates of alkoxylatedtrimethylolpropane, glycerol or pentaerythritol, are also preferred. Theesters of alicyclic diols, such as cyclohexanediol di(meth)acrylate andbis(hydroxymethylethyl)cyclohexane di(meth)acrylate, are furthermoresuitable. Further suitable reactive diluents are trimethylolpropanemonoformal acrylate, glyceryl formal acrylate, 4-tetrahydropyranylacrylate, 2-tetrahydropyranyl methacrylate and tetrahydrofurfurylacrylate.

If the curing of the novel dispersions is carried out not only thermallybut also by means of UV radiation, the novel formulations preferablycontain at least one photoinitiator which is capable of initiating thepolymerization of ethylenically unsaturated double bonds. These include,for example benzophenone and benzophenone derivatives such as4-phenylbenzophenone and 4-chlorobenzophenone, Michler's ketone,acetophenone derivatives, such as 1-benzoylcyclohexan-1-ol,2-hydroxy-2,2-dimethylacetophenone and2,2-dimethoxy-2-phenylacetophenone, benzoin and benzoin ethers, such asmethyl, ethyl and butylbenzoin ethers, benzil ketals, such as benzyldimethyl ketal,2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one,anthraquinone and its derivatives, such as methylanthraquinone andtert-butylanthraquinone and acylphophine oxides, such as2,4,6-trimethylbenzoyldiphenylphosphine oxide,ethyl-2,4,6-trimethylbenzoylphenylphosphinate,ethyl-2,4,6-trimethylbenzoylphenylphosphinate and bisacylphosphineoxides.

The novel dispersions particularly preferably contain at least onephotoinitiator which is selected from phenyl glyoxylic acid and theesters and salts thereof. Compounds of the formula I

where R¹ is hydrogen or C₁–C₁₈-alkyl, are particularly preferred. R¹ ispreferably C₁–C₈ alkyl, in particular methyl, ethyl, propyl, butyl orhexyl.

R² and R³, independently of one another, are each hydrogen, C₁–C₁₈-alkylor C₁–C₁₈-alkoxy.

R² and R³, independently of one another, are each hydrogen.

If at least one of the two radicals R² and R³ is not hydrogen, thephenyl ring is preferably substituted in the paraposition (4-position)relative to the carbonyl group.

Phenylglyoxylic esters of the formula I

where the radicals R⁴, independently of one another, are each a radicalof the formula

R⁵, R⁶ and R⁷, independently of one another, are each H, C₁–C₆-alkyl,which is unsubstituted or substituted by OH, OC₁–C₆-alkyl orOCOC₁–C₆-alkyl, or OH or OC₁–C₆-alkyl, A is C₂–C₆-alkylene or a radicalof the formulae

the radicals R⁸, independently of one another, are each H or COCOR⁴ and

-   A¹ is C₂-C₆-alkylene or

are furthermore particularly preferred.

Such compounds are described in DE-A-198 26 712 and German patentapplication P-199 13 353.0, which are hereby fully incorporated byreference. Preferably, the photoinitiators described above and based onphenylglyoxylic acid derivatives are suitable for exterior applicationssince they undergo little or no yellowing.

The novel dispersions contain the photoinitiators preferably in anamount of from 0.05 to 10, particularly preferably from 0.1 to 8, inparticular from 0.2 to 5, % by weight, based on the total amount of thecomponents a) to i).

The novel dispersions may contain further additives, customary forcoating, such as a leveling agents, antifoams, UV absorbers, dyes,pigments and/or fillers.

Suitable fillers include silicates, for example silicates obtainable byhydrolysis of silicon tetrachloride, such as Aerosil R from Degussa,silica, talc, aluminum silicates, magnesium silicates, calciumcarbonates, etc. Suitable stabilizers include typical UV absorbers, suchas oxanilides, triazines and benzotriazole (the latter is available asTinuvin R grades from Ciba-Spezialitätenchemie) and benzophenones. Thesemay be used alone or together with suitable free radical acceptors, forexample stearically hindered amines, such as2,2,6,6-tetramethylpiperidine, 2,6-di-tert-butylpiperidine orderivatives thereof, e.g. bis(2,2,6,6-tetra-methyl-4-piperidyl)sebacate. Stabilizers are usually used in amounts of from 0.1 to 5.0% byweight, based on the solid components contained in the formulation.

The solids content of the novel aqueous dispersions is preferably fromabout 5 to 70, in particular from 20 to 50, % by weight. Preferreddispersions are those in which isocyanate groups of the compounds ofcomponent a) and, if present, g) have been reacted to an extent of

-   -   from 5 to 98, preferably from 10 to 80, mol % with groups of at        least one compound of component b) which are reactive toward        isocyanate groups,    -   from 0 to 70, preferably from 20 to 60, mol % with groups of at        least one compound of component c) which are reactive toward        isocyanate groups,    -   from 2 to 10, preferably from 4 to 8, mol % with groups of at        least one compound of component d) which are reactive toward        isocyanate groups.

The novel dispersions are particularly suitable for coating substrates,such as wood, paper, textiles, leather, nonwovens, plastic surfaces,glass, ceramic, mineral building materials, such as cement bricks andfiber cement boards, and in particular metals or coated metals.

After exclusively thermal curing, the novel dispersions advantageouslyform films having good performance characteristics, such as good scratchresistance, good resistance to chemicals, good weathering stabilityand/or good mechanical properties. However, they are particularlypreferably suitable for dual-cure applications in which an articlecoated with at least one novel dispersion is subjected to at least onecuring step using high-energy radiation and at least one thermal curingstep.

The present invention furthermore relates to a process for coatingarticles, wherein

-   -   i) an article is coated with a dispersion as described above    -   ii) volatile components of the dispersion are removed for film        formation under conditions under which the thermal initiator h)        substantially still forms no free radicals,    -   iii) if required, the film formed in step ii) is exposed to        high-energy radiation, the film being precured, and then, if        required, the article coated with the precured film being        mechanically processed or the surface of the precured film being        brought into contact with another article, and    -   iv) the film is subjected to thermal final curing.

Steps iv) and iii) can also be carried out in the reverse sequence, i.e.the film can be first thermally cured and then cured using high-energyradiation.

The present invention furthermore relates to coated articles which areobtainable by this process.

The coating of the substrates is effected by conventional processesknown to a person skilled in the art, at least one novel dispersionbeing applied in the desired thickness to the substrate to be coated andthe volatile components of the dispersions being removed. This procedurecan, if desired, be repeated one or more times. The application to thesubstrate can be effected in a known manner, for example by spraying,applying with a trowel, knife coating, applying with a brush, rollercoating or pouring. The thickness of the coating is as a rule from about3 to 1000, preferably from 10 to 200, g/m².

The curing of the films formed on the substrate can, if desired, beeffected exclusively thermally. In general, however, the coatings arecured both by exposure to high-energy radiation and thermally.

If a plurality of coats of the coating material are applied one on topof the other, radiation curing can if required be effected after eachcoating operation.

The radiation curing is effected by the action of high-energy radiation,i.e. UV radiation or daylight, preferably light having a wavelength offrom 250 to 600 nm, or by exposure to high-energy electrons (electronbeams; from 150 to 300 keV). The radiation sources used are, forexample, high-pressure mercury vapor lamps, lasers, pulsed lamps (flashlight), halogen lamps or excimer lamps. The radiation dose usuallysufficient for crosslinking in the case of UV curing is from 80 to 3000mJ/cm².

The exposure to radiation can, if required, also be carried out in theabsence of oxygen, for example under an inert gas atmosphere. Suitableinert gases are preferably nitrogen, noble gases, carbon dioxide orcombustion gases. Furthermore, the exposure to radiation can be effectedby covering the coating material with transparent media. Transparentmedia are, for example, plastic films, glass or liquids, e.g. water.

In a preferred process, the curing is effected continuously by movingthe substrate treated with the novel formulation past a radiation sourceat constant speed. For this purpose, it is necessary for the curing rateof the novel formulation to be sufficiently high.

It is possible to make use of this different time variation of thecuring in particular when the coating of the article is also followed byanother processing step in which the firm surface comes into directcontact with another article or is mechanically processed.

The advantage of the novel dispersions is that the coated article can befurther processed immediately after the radiation curing since thesurface is no longer tacky. On the other hand, the precured film isstill so flexible and extensible that the article can still be deformedwithout the film flaking or tearing.

Even if no deformation of the article is intended, the dual-cure processmay prove advantageous because the articles provided with precured filmcan be particularly easily transported and stored, for example instacks. In addition, the dual-cure process has the advantage that thecoating materials can be chemically postcured in dark parts (parts notaccessible to the radiation) and hence adequate material properties canstill be achieved independently of the exposure to radiation.Furthermore, spray mist deposits cure without tack and without omission.

The present invention furthermore relates to the use of a dispersion, asdescribed above, for coating substrates of metal, wood, paper, ceramic,glass, plastics, textile, leather, nonwovens or mineral buildingmaterials.

The novel dispersions are particularly preferably suitable as or inexterior coatings, preferably of buildings or parts of buildings, roadmarkings or coatings on vehicles and aircraft. In particular, the noveldispersions are used as or in automotive clear coat(s).

The nonrestricting examples which follow illustrate the invention.

EXAMPLES

Unless stated otherwise, parts and percentages are by weight.

The scratch resistance was assessed in an abrasion test in which 50double strokes were performed with a Scotch-Brite fabric under a weightof 750 g. The degree of scratching was determined by determining thedecrease in gloss.

Example 1 1a) Preparation of allophanate from hexamethylene diisocyanateand 2-hydroxyethylacrylate:

Hexamethylene diisocyanate was mixed with 40 mol % (based on theisocyanate) of 2-hydroxyethyl acrylate while blanketing with nitrogenand was heated to 80° C. After the addition of 200 ppm by weight (basedon diisocyanate) ofN,N,N-trimethyl-N-(2-hydroxypropyl)ammonium-2-ethylhexanoate, thetemperature was slowly increased to 120° C. and this reactiontemperature was maintained. When the mixture had an isocyanate contentof 13.5% by weight, the reaction was stopped by adding 250 ppm by weight(based on diisocyanate) of di(2-ethylhexyl) phosphate. The reactionmixture was then freed from unconverted hexamethylene diisocyanate in athin-film evaporator at 135° C. and 2.5 mbar. After the distillation,the product had an NCO content of 13.5% by weight and a viscosity of 810mPas at 23° C.

1b) Preparation of the aqueous polyurethane dispersion:

100 parts of the allophanate from example 1a), 12.4 parts of decanediol,0.13 part of 2,6-di-tert-butyl-p-cresol, 0.1 part of hydroquinonemonomethyl ether and 0.03 part dibutyltin dilaurate were initially takenin a stirred kettle. 9.9 parts of hydroxyethyl acrylate were then addedand the resulting mixture was stirred for 3 hours at 70° C. After theaddition of 2 parts of thioglycolic acid and 1.1 parts of methanol,stirring was continued for a further 4 hours at 70° C. and the reactionbatch was then cooled. After the addition of 2.3 parts of triethylamine,the resulting product was dispersed in water.

Example 2 Preparation of an aqueous polyurethane dispersion withincorporated thermal initiator:

The procedure was as in example 1, the amount of methanol being replacedby benzopinacol.

Production and Testing of Films:

The dispersions of examples 1 and 2 were applied to various substrates,without the addition of an initiator and after addition of a thermalinitiator and, if required, of a photoinitiator, as stated in table 1,so that coating films having a thickness of about 40 mm resulted. Thefilms were dried overnight in the air at room temperature and were thenheated for 15 minutes at 60° C.

For assessing the purely thermal curing of the formulation to which athermal initiator was added, the scratch resistance after curing for 30minutes in a drying oven at 150° C. is shown in table 1. Furthermore,the films of the formulations to which a photoinitiator was additionallyadded, were subjected to radiation curing on a conveyor belt at a speedof 10 m/min using two UV lamps (80 W/cm) and subsequent thermal curingfor 30 minutes in a drying oven at 150° C. The results of the scratchresistance test are likewise shown in table 1.

TABLE 1 Scratch resistance Dispersions from (decrease in Example No.Initiator gloss/%) 1 (Comparison) None 90 1 4% by weight of tert-butyl46 pivalate 1 3% by weight of Irgacure ® 24 I500¹⁾/5% by weight ofbenzopinacol (separate) 2 0.8% by weight of benzo- 40 pinacol(incorporated)²⁾ 2 0.8% by weight of benzo- 32 pinacol (incorporated) 3%by weight of Irgacure ® 1 3% by weight of Irgacure ® 32 I500/5% byweight of dibenzylperoxide 1 3% by weight of Irgacure ® 43 I500/5% byweight of tert-butyl perbenzoate 1 3% by weight of Irgacure ® 32 I500/5%by weight of (phenylsulfonyl) acetic acid methyl ester ¹⁾Mixtures of1-hydroxycyclohexyl phenyl ketone and benzophenone (from Ciba) ²⁾1.1parts by weight, based on components of the polymer

1. A curable aqueous polyurethane dispersion, comprising: a) at leastone compound having at least two free isocyanate groups, at least oneallophanate group and at least one C═C double bond capable of freeradical polymerization, wherein a carbonyl group or an oxygen atom inthe form of an ether function is bonded directly to the double bond, b)at least one compound having at least one group reactive towardisocyanate groups and having a C═C double bond capable of free-radicalpolymerization, c) optionally at least one compound having at least twogroups reactive toward isocyanate groups and selected from the groupconsisting of hydroxyl, mercapto, primary secondary amino groups, andcombinations thereof, d) at least one compound having at least one groupreactive toward isocyanate groups and having at least one acid group, e)at least one basic compound for neutralizing or partially neutralizingthe acid groups of the compound d), f) optionally at least one compounddifferent from b), d) and e) and having only one group reactive towardisocyanate groups, g) optionally at least one polyisocyanate differentfrom a), h) at least one thermal initiator, i) optionally, one or morefurther additives selected from the group consisting of reactivediluents, photoinitiators and conventional coating additives, and k)water.
 2. The dispersion as claimed in claim 1, wherein the component a)is of formula IOCN—R¹

R²—C(O)—R²—R¹

_(n)NCO  (I) where n is an integer from 1 to 10, R¹ is a divalentaliphatic or alicylic C₂- to C₂₀-hydrocarbon unit or an aromatic C₆- toC₂₀-hydrocarbon unit, R² in each repeating unit may be —NH— or

wherein R³ is a radical derived from an alcohol A by abstraction of theH atom from the alcoholic hydroxyl group of the alcohol A, the alcohol Aadditionally having at least one C═C double bond capable of free-radicalpolymerization and a carbonyl group or an oxygen atom bonded by an etherbond directly to the double bond.
 3. The dispersion as claimed in claim1, wherein the thermal initiator h) has a half-life at 60° C. of atleast one hour.
 4. The dispersion as claimed in claim 1, wherein thecomponent h) comprises at least one compound which has at least onegroup reactive toward isocyanate groups and is capable of liberatingfree radicals thermally even after reaction with a compound containingisocyanate groups.
 5. The dispersion as claimed in claim 1, wherein thecomponent h) comprises benzopinacol or a derivative thereof.
 6. Thedispersion as claimed in claim 1, wherein the component h) comprises an(arylsulfonyl) acetic acid alkyl ester.
 7. The dispersion as claimed inclaim 1, wherein the isocyanate groups of the compounds of component a)and, if present, g) are reacted to an extent of from 5 to 98 mol % withgroups of at least one compound of component b) which are reactivetoward isocyanate groups, from 0 to 70 mol % with groups of at least onecompound of component c) which are reactive toward isocyanate groups,from 2 to 10, mol % with groups of at least one compound of component d)which are reactive toward isocyanate groups.
 8. A process for coating anarticle, comprising i) coating an article with a dispersion as claimedin claim 1, ii) removing volatile components of the dispersion for filmformation under conditions under which the thermal initiator h)substantially forms no free radicals, iii) optionally, exposing the filmformed in step ii) to high-energy radiation to precure the film, andthen, optionally, coating the article with the precured film, whereinthe article coated with the precured film is mechanically processed orthe surface of the precured film is brought into contact with anotherarticle, and iv) subjecting the film to thermal final curing.
 9. Acoated article obtained by the process as claimed in claim
 8. 10. Thedispersion as claimed in claim 7 wherein the isocyanate groups of thecompounds of component a) and, if present, g) are reacted to an extentof from 10 to 80 mol % with groups of at least one compound of componentb).
 11. The dispersion as claimed in claim 7, wherein the isocyanategroups of the compounds of component a) and, if present, g), are reactedto an extent of from 20 to 60 mol % with groups of at least one compoundof component c).
 12. The dispersion as claimed in claim 7, wherein theisocyanate groups of the compounds of component a) and, if present, g)are reacted to an extent of from 4 to 8 mol % with groups of at leastone compound of component d).
 13. A method comprising: coating asubstrate with the dispersion as claimed in claim
 1. 14. The method ofclaim 13, wherein the substrate comprises metal, wood, paper, ceramic,glass, plastic, textile, leather, nonwoven or a mineral buildingmaterial.
 15. A method comprising coating a surface with the dispersionas claimed in claim 1, wherein the surface is an exterior coating. 16.The method of claim 15 wherein the surface is a surface of a building, aportion of a building, a road marking, a coating on a vehicle or acoating on an aircraft.
 17. An automotive clear coat comprising thedispersion as claimed in claim
 1. 18. A curable aqueous polyurethanedispersion, comprising: a) at least one compound having at least twofree isocyanate groups, at least one allophanate group, and at least oneC═C double bond capable of free radical polymerization, a carbonyl groupor an oxygen atom in the form of an ether function directly bonded tothe double bond, b) at least one compound having at least one groupreactive toward isocyanate groups and having a C═C double bond capableof free-radical polymerization, c) optionally at least one compoundhaving at least two groups reactive toward isocyanate groups andselected from the group consisting of hydroxyl, mercapto, primarysecondary amino groups, and combinations thereof, d) at least onecompound having at least one group reactive toward isocyanate groups andhaving at least one acid group, e) at least one basic compound forneutralizing or partially neutralizing the acid groups of the compoundd), f) optionally at least one compound different from b), d) and e) andhaving only one group reactive toward isocyanate groups, g) optionallyat least one polyisocyanate different from a), h) at least one thermalinitiator, i) optionally, one or more further additives selected fromthe group consisting of reactive diluents, photoinitiators andconventional coating additives, and k) water.